This invention relates to reception of GPS and other signals via an array antenna subject to array factor distortions and which may be arranged to provide anti-jam processing to reduce jamming and interference.
Global Positioning System (GPS) signal receiving systems must accurately process received signals in order to acquire and maintain lock on satellite signals to provide maximum performance in position determination. It has been recognized that for an airborne GPS system the quality of the received signal may be affected by platform dynamics relative to the signal source (e.g., the satellite transmitting the GPS signal being received). Suggestions for augmenting a GPS receiving system for this purpose have included the use of position aiding or correction information derived from an inertial guidance system, as well as velocity aiding or correction information from the same source.
Suggestions have also been made for use of anti-jam antennas for reception of GPS signals to mitigate effects of intentional or unintentional interference signals, including jamming or other signals which may affect reception of GPS signals. In this context, a multi-element controlled reception pattern antenna (CRPA) system may be used with a GPS system for reception of GPS signals. This type of array antenna system may employ adaptive processing to automatically steer a receiving pattern null to the incident angles of one or more incoming interference signals in order to reduce or eliminate reception of such signals and thereby improve reliability of signal reception during operation of a GPS system. Such an antenna pattern which has been modified (e.g., via adaptive processing to form one or more nulls positioned at particular interference signal incidence angles) may be termed an “adapted antenna pattern”.
It has been determined that an adapted antenna pattern (e.g., as provided via use of a CRPA system and having a non-uniform reception pattern) has reception characteristics which inherently vary with changes in the arrival or incidence angle of incoming signals. Such reception characteristics may be termed “array factors”. As a result, any change in the attitude (roll, pitch, yaw) or position of the antenna (which may also have a velocity or acceleration) may cause an incoming signal to have a different or varying incidence angle relative to the antenna pattern and thereby be subjected to a differing reception characteristic. This differing reception pattern may subject an incoming signal to variations in amplitude, phase and group delay and other effects during signal reception. Such effects may be termed “array factor distortions”.
Also, an array antenna located at a fixed or land mobile position may provide an antenna pattern which, while not an adapted pattern, is subject to non-uniformities or variations in reception characteristics with changes in the incident angle of a received signal. While such variations (e.g., array factor distortions) may be of only negligible significance in many applications, in high performance GPS, broadband or other applications such distortions may affect or limit system performance. As an example, a differential GPS (DGPS) installation at an accurately determined fixed position in the vicinity of an airport may receive GPS signals and provide error correction signals to aircraft for improved accuracy of aircraft position determination. However, such accuracy may be limited by array factor distortions introduced by an array antenna employed for reception of GPS satellite signals by the DGPS installation. Array factor distortions may also affect operation in other applications (GPS, broadband, etc.) involving fixed, movable or other array antenna configurations.
The effects or impact of array factor distortions may be relatively unimportant in many applications even where CRPA systems are involved. However, effects of such distortions resulting from signal reception via an antenna system providing an adapted antenna pattern may undesirably affect achievement of high performance objectives. Attention will be directed to the example of an airborne GPS system which may be employed for vehicle guidance. GPS systems whose performance may be undesirably affected by such reception characteristic variations resulting from signal reception via an adapted antenna pattern may include airborne, vehicle mounted, stationary and other GPS systems.
Accordingly, objects of the present invention are to provide new and improved antenna systems and methods, as well as such systems and methods which may be arranged to mitigate effects of array factor distortions associated with signal reception via adapted and other antenna patterns.